Field of the Invention
The present invention relates to a thermostat, and particularly relates to a device for preventing the abnormal rise in the temperature of a fixation unit for thermally fixing a toner image on the surface of a carrier in a dry electronic copying machine.
Description of the prior art
When a toner image transferred from a photosensitive member to copying paper is passed through a fixation unit, the grains of the toner of the image are bonded to each other and to the copying paper by heat, pressure, a solvent or the like, so that the toner image is fixed on the copying paper. For the fixation, the optimal one of various fixation methods is selected in consideration of the speed of copying, the consumption of electric power, the volume, maintenance and performance of the copying machine and so forth.
With a roller fixation method, heat energy is imparted to the toner and the copying paper through thermal conduction as the copying paper carrying the toner image is passed through between a heating roller and a pressure roller. At that time, heat is directly transmitted from the surface of the heating roller to the copying paper and pressure is applied to the paper by the pressure roller, so that the toner image is fixed on the paper. The heating roller is made of a material high in heat resistance, parting property and mechanical strength. The pressure roller has a surface layer of appropriate elasticity, and performs fixation nipping. the surface layer needs to have the same quality as the heating roller.
A fixation unit for use in the roller fixation method has advantages that the heat efficiency of the unit is as high as 60 to 80%, the unit can be applied to a high-speed copying machine and the safety of the unit is high. For that reason, the fixation unit is widely used in practice.
In a fixation unit for carrying out a radiant fixation method, heat energy is imparted to the toner and the paper by radiant heat transmission to fix the toner image on the paper. For such fixation, an infrared lamp, a xenon flash lamp or the like is used as a heat source. In the radiant fixation method, the degree of the fixation depends on the shade of the toner image. It is typical that the degree of the fixation is high for the high shade areas of the toner image, but is low for the low shade areas thereof because many of the grains of the toner are isolated from each other in the low-shade portion of the toner image, so as not to absorb sufficient heat. A fixation unit for exercising the radiant fixation method has advantages that the construction of the unit is simple, the unit can be made compact, the cost thereof is low and the warm-up time thereof is short. The fixing property of the fixation unit employing the xenon flash lamp is higher than that of the fixation unit employing the infrared lamp. However, the power supply of the fixation unit employing the xenon flash lamp is heavier and more expensive than that of the fixation unit employing the infrared lamp.
In another pressure fixation method which recently has been receiving more and more attention, pressure is applied to a toner made of a pressure-sensitive substance such as paraffin, wax and a rubber-like soft polymer to perform fixation. Normally, a pressure of 20 to 40 kg/cm.sup.2 is applied to the toner to lower the viscosity thereof and imprequate the toner in to the fibers of copying paper.
A fixation unit for exercising this pressure fixation method has advantages that the unit does not need a heat source and warm-up period. However, this fixation unit has a still uneliminated disadvantage that the fixation of the toner often is not sufficiently performed.
Among the other above-described fixation methods, a heating fixation method is widely use. In a well-known fixation unit for exercising the heating fixation method, a prescribed pressure is applied between a heating roller and a facing pressure roller, and a carrier having an unfixed toner image is passed through between the rollers so that the toner image is fixed on the carrier. The fixation unit has advantages that the electric power consumption thereof is less than that of other heating fixation units and fires are less likely to occur due to the jamming of paper (the carrier) in this fixation section. The heating roller has a surface coating layer of a heat-resisting parting substance such as polytetrafluoroethylene (commercially named Teflon), HTV silicone rubber and RTV silicone rubber. The pressure roller is made of a heat-resisting elastic material such as silicone rubber and fluorine rubber, so that a certain contact width (usually referred to as nip) is established when the pressure roller and the heating roller are put in pressure contact with each other.
In this well-known heating fixation unit, the temperature of the surface of the heating roller needs to be raised from the room temperature level to a level necessary for the fixation. For that reason, copying cannot be started immediately after electric power is applied to a copying machine. Therefore, it takes some time to warm up the heating roller. The time normally is about 1 to 10 minutes. This is a disadvantage for the fixation unit.
In order to eliminate the disadvantage, it has been made possible to reduce the heat capacity of the heating roller and apply as heavy an electrical current as possible at first to shorten the warm-up time to 0.5 to 1 minute. If the warm-up time is shortened as mentioned above, the temperature of the heating roller is sharply raised. The rate of the rise in the temperature of the heating roller in that case is as high as 3.degree. to 10.degree. C./sec. Therefore, the fixation unit needs to be provided with an abnormal temperature rise prevention device. When the pressure roller is heated above a control level due to improper operation of a temperature control circuit, disconnection, short-circuiting or wrongly set position of a sensor or the like, it is detected by the device so as to stop the application of electricity to a heater to eliminate the possibility of occurrence of a fire or the like.
As a conventional abnormal temperature rise prevention device, a thermostat, a temperature control fuse or the like is connected in series with a heater. When the rise in the temperature of the heating roller is sharp, the response of the thermostat, the temperature control fuse or the like is affected so that the device sometimes does not accurately operate. At that time, even if the temperature of the heating roller has become abnormally high, the device cannot detect the rise in the temperature until the temperature goes up to a paper burning level. This is a serious problem, which is likely to happen during the warm-up of a copying machine at the start of a copying operation.
Since the electrically charged portion of each of most of such temperature control fuses is exposed, the fuse needs to be placed at an appropriate distance from the heating roller. The thermal conductivity of the other temperature control fuses, whose electrically charged portions are not exposed, is low because of the presence of an electric insulator. Therefore, generally speaking, the response of the thermostat to the abnormal rise in the temperature is better than that of each of the temperature control fuses.
FIGS. 5 and 6 show constructions of widely used thermostats 1a. Each of the thermostats 1a has a housing 10 comprising a cylindrical portion 12 and a bottom portion 14. A disk holder 30a is secured to the end of the cylindrical portion 12. A fixed contact 16 and a movable contact 18 corresponding thereto are provided in the housing 10. The movable contact 18 is supported by a spring plate 19, which is pushed by a moving pin 24a to turn the thermostat on or off. A fixed cap 40a is provided on the housing 10 and covers the disk holder 30a. The central portion of the fixed cap 40a is formed as a heat reception plate 46 to transmit heat to a bimetal disk 20. For that reason, the fixed cap 40a is made of a metal, such as aluminum or stainless steel, which has a high thermal conductivity and is unlikely to rust. When the temperature of the bimetal disk 20 of the thermostat 1a has exceeded a set level, the form of the bimetal disk changes from concave to convex, to displace the moving pin 24a toward the movable contact 18. This pushes the prescribed portion of the spring plate 19 to separate the movable contact 18 from the fixed contact 16, thus turning off the thermostat.
The bimetal disk 20 of such a conventional thermostat is heated by radiant energy from a heating roller and the conventional energy of the atmosphere, but much of the heat energy absorbed by the bimetal disk 20 is conducted to other members, such as the disk holder 30a and the moving pin 24a, which are located in contact with the bimetal disk. Thus, the amount of heat energy which acts to put the bimetal disk in to action is very small. For that reason, the rise in the temperature of the bimetal disk 20 to reach the set level is generally very slow. This deteriorates the thermal response of the thermostat 1a.
Also, the dessipation of heat from the bimetal disk 20 can be broadly divided into three categories. The first category is the heat transferred from the bimetal disk 20 to the disk holder 30 located in contact with the peripheral portion of the disk. The second category is the heat passing from the bimetal disk 20 to the fixed cap 40a. The third portion is the heat transmitted from the disk 20 to the moving pin 24a.
As shown in FIGS. 6 and 6A, the peripheral portion of the bimetal disk 20 is located in contact with the butt of the flange 34a of the disk holder 30a at an engaging portion 38a along the entire circumference of the bimetal disk. Therefore, heat transfers from the bimetal disk 20 to the disk holder 30a through the peripheral portion of the disk.
The disk holder of a latest conventional thermostat of the above-described type for high temperature use is not made of a heat-resisting resin, but is made of a ceramic, because the ceramic is cheap and highly heat-resisting. The thermal conductivity of the ceramic is 10 to 500 times higher than that of the heat-resisting resin. Therefore, the quantity of heat which passes from the bimetal disk of the thermostat to the disk holder is even larger.
Before the bimetal disk 20 is put in to action, the fixed cap 40a is in contact with the bimetal disk. For that reason, much of the heat received by the heat reception plate 46 is transmitted to the disk holder 30a and the housing 10 through the peripheral portion of the heat reception plate 46. Therefore, the amount of heat which effectively acts on the bimetal disk is very small.
In the conventional thermostat shown in FIG. 5, the tip of the moving pin 24a is located in contact with the central portion of the bimetal disk 20. Because of this contact, the heat received by the bimetal disk 20 is transmitted through the moving pin 24a.
For the above-described reasons, the thermal response of each of the conventional thermostats is not good.